The present invention relates generally to wireless communication systems and methods, and more specifically to DC offset cancellation for a wireless receiver.
Wireless network technology conveniently allows a mobile user to wirelessly connect to a wired network, such as an enterprise's local area network (LAN). Heterodyne receivers and direct downconversion receivers are generally used to receive wireless transmissions. In a heterodyne receiver, a radio frequency (RF) signal is mixed with a signal from a local oscillator to produce an intermediate frequency (IF) signal. Through various IF stages with mixers, oscillators, and filters, the IF signal eventually is downconverted to a baseband signal for further processing.
Direct downconversion receivers (DCRs) directly down-convert a received RF signal to a baseband signal without additional signal processing at various IF stages. DCRs have a local oscillator operating at the carrier frequency used to downconvert signals to baseband, resulting in a simpler architecture. The downconverted signal is amplified by a gain to achieve optimal signal strength before presentation to a digital detector or demodulator for further processing.
Processing of received signals may be negatively impacted by DC offset. DC offset may occur due to, for example, current leakage from a receiver's local oscillator (LO), signal reflections, temperature changes, and other causes.
An AC-coupled high pass filter can provide a straightforward approach to correct DC offset, with the filter removing low frequency and DC signal components. Such high pass filters are more effective in removing effective DC offset when the corner frequency of the filter is closer to the frequencies carrying information. Unfortunately, filters with high corner frequencies are also more likely to attenuate signals at the information carrying frequencies. Accordingly, some have proposed use of a high pass filter with a corner frequency switchable from a first higher frequency used during initial reception to a second lower frequency during reception of other portions of a signal. Use of such filters, however, may result in a substantially instantaneous change, or distortion, in perceived signal strength and DC offset by downstream processing components.